The present invention relates to compression mechanisms. More particularly, the present invention relates to compression mechanisms for fuel cell assemblies in which the mechanism for securing the assemblies in their assembled, compressed state comprises at least two compression spring sheets which extend under tension between endplates of the fuel cell assembly urging the endplates toward one another.
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers comprising porous, electrically conductive sheet material and an electrocatalyst disposed at each membrane/electrode layer interface to induce the desired electrochemical reaction.
In a PEM fuel cell, the MEA is disposed between two electrically conductive separator or fluid flow field plates. Fluid flow field plates have at least one flow passage formed therein to direct the fuel and oxidant to the respective electrode layers, namely, the anode on the fuel side and the cathode on the oxidant side. In a single cell arrangement, fluid flow field plates are provided on each of the anode and cathode sides. The plates act as current collectors and provide support for the electrodes.
Two or more fuel cells can be connected in series or in parallel to increase the overall power output of the assembly. In series arrangements, one side of a xe2x80x9cbipolarxe2x80x9d plate can serve as the anode plate for one cell with the other side of the plate serving as the cathode plate for an adjacent cell. Such a multiple fuel cell arrangement connected in series is referred to as a fuel cell stack, and typically is held together in its assembled state by tie rods or compression bands and endplates.
A fuel cell stack typically includes manifolds and inlet ports for directing fuel and oxidant streams to the anode and cathode flow field passages respectively. Usually the stack includes a manifold and inlet port for directing a coolant fluid, typically water, to interior passages within the stack to absorb heat generated by the exothermic reaction in the fuel cells. The stack also generally includes exhaust manifolds and outlet ports for expelling fuel and oxidant exhaust streams, as well as an exhaust manifold and outlet port for the coolant stream exiting the stack.
Typically, endplates are placed at each end of the stack to hold the stack together and to compress the stack components together. Compressive force is needed for effecting seals and making adequate electrical contact between various stack components. For various reasons, some resilience is generally desirable in the compression endplate assemblies, for instance to accommodate and compensate for dimensional changes and to maintain compressive force over prolonged periods of time. Examples of various resilient compression endplate assemblies are disclosed in U.S. Pat. Nos. 5,484,666 and 5,789,091.
To reduce the number of component parts, and improve volume efficiency, stack manifolds can be incorporated into compression endplates of fuel cell stacks in an array. For example, U.S. Pat. No. 5,486,430 shows an array manifold integrated into the compression endplates of multiple fuel cell stacks.
In conventional fuel cell designs, the components that make up each fuel cell assembly are compressed and maintained in their assembled state by tie rods. The tie rods extend through holes formed in the peripheral edge portion of the stack endplates and have associated nuts or other fastening means for assembling the tie rods with the stack assembly and springs or other resilient means for urging the endplates toward each other. A fuel cell stack design incorporating internal tie rods which extend between the endplates through openings in the fuel cell plates and membrane electrode assemblies has been described in U.S. Pat. No. 5,484,666.
Use of external tie rods requires that each of the endplates be greater in area than the stacked fuel cell assemblies interposed therebetween, which can increase stack volume and stack weight significantly. This is particularly undesirable in transportation applications using fuel cells. The associated fasteners also increase the number of parts required to assemble a stack.
The use of compression bands to compress fuel cell stacks has been described in U.S. Pat. No. 5,789,091. In the compression band system, at least one compression band circumscribes the first and second endplate assemblies and the interposed electrochemical fuel cell assemblies. The resilient compression assembly urges the first endplate assembly toward the second endplate assembly, thereby applying compressive force upon the fuel cell assembly. The compression assembly for compressing the fuel cell assemblies preferably applies the desired internal compressive force while accommodating changes in fuel cell thickness.
Traditional compression assemblies comprise springs and/or hydraulic pistons, employed either individually or in combination. Springs are often used as a backup to provide a compressive force if the hydraulic piston pressure is lost or inadequate for applying the desired compressive force for efficient and safe fuel cell operation. In either case, ideally the desired compressive force is applied to the fuel cell assemblies over the range of internal pressures expected in an operational fuel cell stack. Unfortunately, the use of a hydraulic piston adds to the complexity of the fuel cell stack and can be a source of unreliability, with potentially adverse consequences if the piston-based compression system fails.
In lieu of hydraulic pistons, some conventional fuel cell stacks use compressed springs in conjunction with a retention device, such as tie rods or compression bands. In response to reductions in the thickness of stack components, the compressed springs expand, to continue to apply compressive force to the fuel cell assembly.
In general, a problem with compressed springs is that as a compressed spring expands, its spring force declines, resulting in a decreasing ability to apply compressive force to the stack components. The decline in spring force can be reduced by using a spring having a very low spring rate. For example, disc springs (sometimes referred to as Belleville springs or Belleville washers) can be made with a spring rate suitable for use in fuel cell compression assemblies.
In conventional fuel cell stacks, the desire to have a low spring rate to accommodate stack component shrinkage is balanced against the need for a very high spring rate to counter the effect of changes in internal stack fluid pressure on internal compressive force. In conventional fuel cells, a compromise is typically made between these two conflicting requirements by applying high pre-compression forces to mechanical compression assemblies and limiting stack fluid pressures.
In the field of fuel cell compression systems, it is desirable to employ a spring device capable of high loading with high deflection. Such requirements can be met with a compression spring sheet as described herein.
In one embodiment, a compression spring sheet comprises a generally planar material rendered resilient by having a plurality of openings therein.
The plurality of openings may comprise internal openings of a first shape and dimension disposed in longitudinal rows, and peripheral openings of a second shape and dimension, wherein at each opposing end of said spring sheet a peripheral opening is disposed at the end of each alternating row of internal openings.
In a further embodiment, each internal opening comprises first and second lobes and each opening is symmetrical about its longitudinal and transverse axes. A plurality of the first lobes in each row are aligned with the second lobes in adjacent rows.
The compression spring sheet material may be a metal, such as, for example, a metal selected from the group comprising aluminum, steel and titanium. The compression spring sheet may further comprise a plastic coating. Certain plastics may also be suitable materials for compression spring sheets.
Upon the application of in-plane tensile force perpendicular to the longitudinal axis of said openings, the compression spring sheet preferably exhibits approximately uniform stress along the edges of the openings. For example, the stress at any point along the edges preferably varies by no more than 15% relative to the mean stress along the edges of the openings. Preferably, the average stress force on the material surrounding the internal openings is approximately equivalent to the average stress force on the material surrounding the peripheral openings.
In one embodiment of a compliant compression mechanism for an electrochemical fuel cell assembly, the assembly comprises a first plate; a second plate; and at least one membrane electrode assembly interposed between the first and second plates, the mechanism comprising at least one compression spring sheet, each spring sheet comprising a generally planar material rendered resilient by having a plurality of openings formed therein, wherein each spring sheet extends between said first and second plates perpendicular to the plane of the at least one membrane electrode assembly, whereby each spring sheet urges the first plate towards the second plate such that compressive force is applied to the at least one membrane electrode assembly.
The plurality of openings may comprise internal openings of a first shape and dimension and peripheral openings of a second shape and dimension, wherein the internal openings are disposed in longitudinal rows extending between opposed ends of the spring sheet. At each end of the spring sheet, a peripheral openings may be disposed at the end of each alternating row of internal openings.
In a further embodiment, the first and second plates may be endplates. A plurality of fuel cell assemblies may be interposed between the first and second plates. In another embodiment, a plurality of fuel cell stacks may be interposed between the first and second plates.
In another embodiment of a compliant compression mechanism, a pair of opposed spring sheets may be joined by at least one band extending perpendicularly to the spring sheets across the face of one of the plates.
The compliant compression mechanism may comprise on each side at least two substantially co-planar compression spring sheets extending between the endplates of a fuel cell assembly. The total area covered by the one or more compression spring sheets disposed on a side of a fuel cell assembly may be less than the total area of such side.